Generally, in a technical field to which a radiation shielding technique is applied, particularly in a case where a radiation therapy targeting at a human body and a measuring operation are performed, countermeasure for lowering an absolute amount of the harmful radiation has been essentially demanded. In order to reply to this demand, the following countermeasures have been devised. Namely, the radioactive rays are effectively irradiated to only a target portion, while the irradiation of the radioactive ray is not performed to portions other than the target portion, and the radiation time is shortened as short as possible.
However, it is essentially difficult to limitedly irradiate the radiation only to the target portion which is an objective portion to be examined by means of the radiation. Therefore, there has been actually taken a countermeasure such that the portions other than the target portion to be examined are covered with a shielding material for shielding the radiation, whereby an object (human body) to be examined is protected from being exposed to the radiation.
Further, also in case of an X-ray room or an X-ray medical examination car (vehicle) with which an X-ray generating apparatus is equipped, for the purpose of preventing the X-ray from leaking from a wall surface of the X-ray room to outside of the room or preventing the X-ray from leaking to outside of the car, there has been taken a countermeasure such that the radiation shielding material is attached to the wall surfaces of the X-ray room or the like. Furthermore, when an X-ray photograph is taken, a doctor and a patient would wear X-ray protection clothes and they engaged in the X-ray examining operation so as to avoid to be exposed to unnecessary X-ray radiation.
As a material for shielding the X-ray which is one kind of the above radiations (radioactive rays), as has been prescribed in Japanese Industrial Standard (JIS Z4806, Z4801), there has been conventionally used mainly a lead (Pb) or a composite material containing the lead. However, a lead component is harmful when the lead component is absorbed into a human body, and the lead component shall demand a lot of attentions in handling or disposal thereof. Namely, the handling of the lead component is required to comply with strict regulations specified in Lead Poisoning Prevention Rules. While, in case of the disposal of the lead component, it is necessary to perform a treatment for securing that elusion or leaking of the lead component to an outer world is securely blocked.
In recent years, in view of the above problems, as disclosed in a patent document 1 (Japanese Patent Publication: No. 2001-83288) and a patent document 2 (Japanese Patent Publication: No. 2002-365393), there have been proposed a countermeasure in which tungsten (W), tin (Sn), antimony (Sb), bithmus (Bi) and compounds thereof are used as the radiation shielding materials taking the place of the harmful lead.
Further, as for the X-ray protection clothes requiring a flexibility for matching an outer shape of the object to be examined, there has been generally used a material which is formed by blending the above materials with resin or rubber to prepare a material mixture, followed by molding the material mixture. In another case where a radiation having a relatively low intensity is used, an acrylic plate or the like are used as a simple countermeasure. On the other hand, in a case where a radiation having a relatively high intensity is used, there has been generally used a plate-shaped radiation shielding material composed of tungsten (W) or the like having a high shielding capability.
However, although the tungsten (W) plate has a high capacity of shielding the radiation, tungsten is a high cost material taking the place of lead (Pb). Further, bithmus (Bi) also has a high radiation-shielding capacity equivalent to that of Pb. However, bithmus is also a relatively high cost material. On the other hand, both antimony (Sb) and tin (Sn) are insufficient in the radiation shielding capacity, so that a thickness of a radiation-shielding sheet becomes thick in order to secure a sufficient shielding capacity, thus resulting in a disadvantage in lacking of mobility during handling the shielding sheet. In addition, it has been suggested that antimony (Sb) has toxic consequences similar to arsenic. In view of the above circumstances, there have been demanded a radiation shielding sheet which is free from an environmental problem, and has a uniform and high radiation shielding capacity and an excellence in economical efficiency.
Patent Document 1: Japanese Patent Application Laid-open Publication No. 2001-83288
Patent Document 2: Japanese Patent Application Laid-open Publication No. 2002-365393
However, in the conventional radiation shielding materials, since the lead or the composite material containing lead had been used as the material for constituting the radiation shielding materials, there had been posed the following problems as described hereinbefore. Namely, such material was harmful when the material was absorbed in a human body, and special attentions must be paid at a time of handling or disposal of the shielding material, thus being lack in safety of the radiation shielding materials.
As a radiation shielding material taking the place of lead, there has been proposed that tungsten (W), tin (Sn), antimony (Sb), bithmus (Bi) and compounds thereof should be used. However, the materials such as tungsten, bithmus and compounds thereof were high cost materials as a material in place of lead, so that a manufacturing cost of the shielding material is disadvantageously increased. In addition, there is arisen a fatal problem such that tungsten, bithmus and compounds thereof were insufficient in shielding capacity in comparison with those of the conventional materials.
The present invention had been achieved to solve the aforementioned problems caused in the conventional prior arts, and an object of the present invention is to provide a radiation-shielding sheet which is free from any environmental problems and safety problems for a human body, and having a highly radiation shielding performance and excellent economical efficiency.